Flows and Transport in Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 107

FIGURE 4.26 Discharge plume with lateral contraction. (From CORMIX image gallery, Available at http://
www.cormix.info/picgal/rivers.php; original source unknown.)
and below the surface, would move faster than the dye on the banks, at the water surface, or at the
bottom. The consequence would be that while the centroid of the dye would move with the average
velocity, some dye would move faster and slower; therefore, as the dye cloud moves downstream, the
concentration cloud would spread out and the peak concentration would be reduced (see Figure 4.14).
If, instead of being completely mixed over the cross section, the dye were injected at the bank,
or at a speciic location within the cross section, then it may be advected downstream at a faster rate
than if it mixed laterally, so it may take a considerable distance downstream for the dye to mix over
the width of the cross section (see Figure 4.26).
Similarly, when two tributaries join, their waters often do not mix completely at the junction,
and they may be maintained separately for considerable distances downstream, as illustrated in
Figure 4.27.
The velocity variations also produce changes in physical habitats, sedimentation rates, and other
hydraulic and biotic river characteristics. For example, mussels and macrophytes may not occur in
some sections of the river where shear stresses due to lows are greatest, but they may survive in
other areas of the river. The variations in velocities and other hydraulic properties are also respon-
sible for the formation of pools, rifles, bars, meanders, and other important physical characteristics
of rivers and streams. Therefore, assuming that rivers are one-dimensional in terms of hydraulics,
water quality, and ecology is misleading at best.
4.6 METHODS FOR DETERMINING INSTREAM FLOW
REQUIREMENTS: ENVIRONMENTAL FLOWS
The streamlow requirements will vary with what we are trying to protect. The streamlow rates
for hydraulic design are usually some rare high-low event, such as the 100-year low. If the conse-
quences of structural failure are signiicant, and we are trying to prevent loss of life, then even more
stringent conditions may be used in the design, such as a maximum probable storm. The streamlow
requirements for habitats will vary with the location and the organisms, and the method used will
vary from individual low conditions to the stream hydrograph.
The instream low can be deined as the amount of water lowing past a given point within a
stream channel during 1 second (Estes 1994), while the instream low requirements have most com-
monly been based on the habitat requirements for some species or groups of ish. Historically, these
requirements have been expressed in terms of some minimum low, such as a minimum low that
permitted water users or dams are required to release or pass, the nondepletable low, or other appro-
priate instream low limits. For example, minimum instream lows have commonly been established